Atomic weights of practically all the elements are very nearly whole no. and not always a whole no. This is because

An element consists of several isotopes, each with the same atomic number or number of protons, but with different numbers of neutrons. Usually one isotope is common and the others consist of a small percentage of the total.

Listings of atomic weights or unified atomic mass units usually average the isotopes of an element. Thus, even if the old integer system for atomic weights held, the number still wouldn't be simple. For example, the atomic weight of Carbon-12 is 12 amu. But Carbon consists of C-12, C-13, and C-14 isotopes. Thus the average atomic weight of Carbon is 12.01 amu.

 

Source:
http://www.school-for-champions.com/science/atomic_weight.htm

 

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